SIKHOTE-ALIN NATURE MONUMENT, Russia  It's time for Volya the tiger to open wide and say "ahhh" so experts can see how the 1 1-2-year-old cub is recovering from an operation that saved her life after she was shot in the head by poachers who killed her mother.

Volya is one of the most endangered animals on the planet -- an Amur tiger. More commonly known as Siberian tigers, the massive cats roam the snowy mountainous terrain of Russia's Far East and northeast China.

Along with other endangered animals and plants, the tigers are part of a unique ecosystem that faces a new threat: a toxic benzene slick headed down the Amur River after an explosion at a Chinese chemical factory.

The international environmental group WWF considers the Amur area a "high-priority conservation region," home to endangered tigers and leopards. It has expressed concern about the effects of the Chinese spill here and called for stricter monitoring on industrial chemicals.

The Amur itself is home to dozens of types of fish, and the area is a habitat for bears and musk deer.

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But the animal that most symbolizes the Far East is the Siberian tiger, a common feature in regional government emblems and a focus of local and international preservation efforts.

Volya is one of two tigers in the care of the Wild Animals Rehabilitation Center at the Sikhote-Alin Nature Monument, a 5,200-hectare (12,850-acre) preserve located 135 kilometers (85 miles) southeast of the regional capital Khabarovsk.

Nearly dead when she was brought here with a shattered jaw in January, her struggle to survive earned her the name Volya, which means "will" in Russian -- as in "will to live," said the center's director Eduard Kruglov.

Kruglov's late father Vladimir, a former tiger hunter who captured more than 40 animals for zoos and circuses, founded the animal center 12 years ago with its first inhabitant, Volya's neighbor Lutiy -- another Siberian tiger whose tame nature in the presence of visitors belies the meaning of his name: "savage beast."

Lutiy also was brought to the center after falling victim to poachers; he received a titanium tooth implant to help him eat his weekly diet of 200 kilograms (440 pounds) of meat.

About 430-540 Siberian tigers remain in the wild in Russia, according to a tiger census conducted this year by the New York-based Wildlife Conservation Society and other groups, who sent some 1,000 field workers to remote areas to count paw prints in the snow. The tigers were once even closer to extinction -- in the 1940s, there were only 40 wild tigers, according to WWF.

The benzene spill likely won't directly affect the tigers unless they drink from the tainted river itself, and that is unlikely, said John Goodrich, field coordinator for society's Siberian Tiger Project.

However, Kruglov noted that the entire ecosystem could be affected by the spill because of interconnected food chains: bears and birds eat fish, and the tigers could be sickened by eating birds. Fish traveling the Amur and back down tributaries could spread chemicals across the region.

"Everything is connected: water, fish, animals and people," said Andrei Dolin, head of the Khabarovsk zoo -- home to two Siberian tigers -- who also was helping with Volya's checkup.

Still, the largest threats tigers face now are hunters who sell their pelts or other body parts for use in traditional Chinese medicines and encroachment on their vast habitat by human development and logging.

"Communism was good for tigers because the borders were closed and there was no market for tigers," said Goodrich, who was visiting the animal center Monday to assist in Volya's exam.

The tiger population has apparently stabilized, with this year's count showing the numbers of animals the same as in 1996, a positive sign attributed to anti-poaching efforts and the creation of protected areas.

Getting a tiger to say "ahh" requires more than the usual persuasion. For the exam, a tranquilizer dart calms Volya so she can be removed from her cage and laid peacefully on a table -- eyes wide open, tongue hanging out of her mouth, her single remaining canine tooth showing.

Volya's handicapped mouth means she'll never be freed into the wild where she'd be unable to survive, the same as with Lutiy. The center has previously released more than 100 bears -- targeted by poachers for their paws, considered a delicacy in China -- after nursing them to health.

But Kruglov was pessimistic about the future faced by the animals in the wake of the latest chemical spill, with the Amur already labeled by environmentalists as heavily polluted.

"There are too many factories in China," he said. This latest accident "won't be the last."